You defeated the Pokémon League! Now you are considered almost the best trainer on the game and now for a bigger challenge, Gamefreak makes trainers of the post-game to be stronger than the Pokémon League. If you see, this happens in all games.
In Pokémon Black a great place for train is defeating Audino at Giant Chasm Plains (85 % Walk - Shaking Spots All) with level between 52 - 55. Hold a Exp. Share /Lucky Egg and defeat as much you can.